1. Field of the Invention
This invention relates to digital communication systems, and in particular to an apparatus and method for detecting the onset or presence of a frequency shift keyed signal which alternates regularly between two unknown frequencies with a known frequency shift and a known key rate.
2. Description of the Prior Art
The frequency shift keyed (FSK) signal is one way of modulating a sine wave carrier to convey binary information in which the carrier switches or alternates between two predetermined frequencies. This may be accomplished by either frequency modulating one sine wave oscillator or by switching between two oscillators. In the latter case the two oscillators may be locked in phase in which case there is said to be phase coherence between successive pulses of each frequency; or the oscillators may not be locked in phase in which case the successive pulses of each frequency are said to be non-coherent. The two frequencies may alternate regularly at what is called the shift rate or key rate to produce a simple FSK signal. Or the frequencies may alternate irregularly to produce a coded FSK signal. In either case the alternating frequencies can be considered as deviating equally above and below an average, center, or carrier frequency with a total frequency deviation referred to herein as the frequency shift or shift frequency.
Frequency shift keyed communication signals may be used in communication systems to alert the start or end of a transmission, to synchronize remote equipment, or to transmit data. Under these circumstances detection of the onset of such an FSK signal, either by a communication receiver or by an intercept receiver, is a necessary system function.
In some situations, the transmission carrier frequency of the FSK signal may not be known at the receiver due to doppler shift, for example, or due to drift or wandering of the carrier frequency at the transmitter. Unknown carrier frequency makes the onset detection of these signals more difficult. For example, in a well known process known as synchronous detection the incoming signal, alternating between frequencies f.sub.1 and f.sub.2, is split into two signal paths. Each signal path is then multiplied by respective locally generated sine waves, one being the same frequency and phase as f.sub.1, and the other the same as f.sub.2. The resultant product terms are then passed through low pass filters to eliminate second harmonic terms. A shortcoming of the synchronous detection process is that the average carrier frequency and frequency shift, or frequencies f.sub.1 and f.sub.2, must be known a priori. Any drift or wandering of the carrier frequency significantly deteriorates system performance.
In another prior art detection scheme, known as noncoherent envelope detection, the incoming signal is split into two signal paths through a pair of narrow band filters, one filter centered at f.sub.1 and the other filter centered at f.sub.2. The outputs of the two filters are then individually envelope detected and compared to determine whether one binary symbol or the other was transmitted. Again the same shortcoming is evident. If the incoming frequencies drift or wander outside the filter pass bands, no signal will be detected.
One method has been proposed for detecting the onset of a signal of this type which takes advantage of the fact that any doppler shift or frequency wandering will equally affect both f.sub.1 and f.sub.2. While the carrier frequency may be unknown, the frequency shift and key rate are still known. The prior art technique uses a digital computer and the well known fast Fourier transform algorithm (FFT) to transform the incoming signal into the Fourier coefficients of its frequency spectrum. The computer then searches this spectrum for coincidences of signal which are separated by the frequency shift. One disadvantage of this system is that it requires a rather large computer and considerable memory space.
Another problem with this prior art detection scheme is encountered when the environment is noisy or when other unwanted signals are present, particularly if these unwanted signals are separated in frequency by the frequency shift. For example, the prior art detector would be unable to distinguish between the incoming frequency shift keyed signal and a pair of continuous wave (CW) signals at any two frequencies separated by the frequency shift value.
The present invention overcomes the foregoing disadvantages of the prior art and provides a highly effective detection system for frequency shift keyed signals whose carrier frequencies are unknown. The invention achieves these advantages without expensive computers or complex algorithms.